build system: update std.Progress.Node for long-lived children

lib/std/Build/Step.zig

@@ -361,6 +361,7 @@ pub fn addError(step: *Step, comptime fmt: []const u8, args: anytype) error{OutO
 pub const ZigProcess = struct {
     child: std.process.Child,
     poller: std.io.Poller(StreamEnum),
+    progress_ipc_fd: if (std.Progress.have_ipc) ?std.posix.fd_t else void,
 
     pub const StreamEnum = enum { stdout, stderr };
 };
@@ -375,6 +376,7 @@ pub fn evalZigProcess(
 ) !?[]const u8 {
     if (s.getZigProcess()) |zp| {
         assert(watch);
+        if (std.Progress.have_ipc) if (zp.progress_ipc_fd) |fd| prog_node.setIpcFd(fd);
         return zigProcessUpdate(s, zp, watch);
     }
     assert(argv.len != 0);
@@ -404,6 +406,7 @@ pub fn evalZigProcess(
             .stdout = child.stdout.?,
             .stderr = child.stderr.?,
         }),
+        .progress_ipc_fd = if (std.Progress.have_ipc) child.progress_node.getIpcFd() else {},
     };
     if (watch) s.setZigProcess(zp);
     defer if (!watch) zp.poller.deinit();
@@ -435,6 +438,8 @@ pub fn evalZigProcess(
         try handleChildProcessTerm(s, term, null, argv);
     }
 
+    // This is intentionally printed for failure on the first build but not for
+    // subsequent rebuilds.
     if (s.result_error_bundle.errorMessageCount() > 0) {
         return s.fail("the following command failed with {d} compilation errors:\n{s}", .{
             s.result_error_bundle.errorMessageCount(),